This invention relates to a gas-fired burner for a water heating system and, more specifically, to a burner that operates over a broad modulation range having excellent stability with a reduced length combustion region.
A major problem regarding water heating systems relates to the overall size of the installation, which depends in part on the volume of the combustion device and associated heat exchanger. A smaller installation size of the system is desirable to conserve space, as long as the system operates efficiently and with low emission of pollutants.
For thermodynamic efficiency, the water heating system may employ a modulating fuel gas burner. Efficient water heating systems may anticipate their load and respond to changes by controlling the flow of air and fuel gas to the burner. The higher the turndown ratio that can be achieved with the modulating burner, the more efficiently the water heating system can be operated. However, a high turndown ratio must be achieved with a system that also has a high maximum energy release, for example, up to and exceeding about two million BTU per hour for commercial water heating systems.
Another problem relates to the degree of carbon monoxide and other emissions. To reduce emissions, adequate mixing of air and fuel is needed. Also, some emissions tend to increase with increasing flame temperature and length. Conventional power-driven burners require a long combustion region to provide complete burning at low emissions.
To avoid the losses of efficiency and reduce emissions, water heating systems typically have a long combustion region followed by a heat exchanger. While the long combustion region provides adequate residence time for mixing and complete combustion, it also suffers radiant transfers from the flame or combusting mixture directly to the walls. Further, a long flame and combustion region increases the overall size of the water heating system dramatically. Commercial burners operating at about two million BTU per hour typically require a combustion region of at least fifty inches in length, exacerbating the problem of limited installation space.
Turndown ratios of about 4:1 or more can only be achieved with power-driven burners that utilize forced air for the combustion mixture. Such burners typically require a long combustion region for adequate mixing of fuel gas and air, complete burning, and reduced emissions.
One way to reduce the overall package size and installation size of the water heating system without sacrificing efficiency is to employ a counterflow design of the water in the heat exchanger relative to the flue gas. For example, U.S. Pat. No. 5,365,887 describes a counterflow water heating system that reduces the overall package size by reducing the heat exchanger size. In another example, U.S. Pat. No. 4,735,174 describes a counterflow water heating system that increases efficiency of heat exchange. In another example, a condensing mode counterflow water heating system is described in U.S. Pat. No. 5,687,678 which employs nested heat exchanger coils.
Another important means to reduce overall size of the water heating system and improve efficiency is to reduce the length of the combustion region and chamber. A smaller combustion region requires that the length of the flame or combusting mixture be reduced, while still allowing efficient heat exchange and complete combustion. For example, U.S. Pat. No. 4,884,555 describes a detached flame swirl burner for a water heater having a short, turbulent flame. However, the combustion region is well above the burner assembly, adding length to the system.
In further examples, gas-fired nozzle mix burners for a water heating system are described in U.S. Pat. Nos. 4,852,524 and 5,881,681. However, these systems operate with a long flame and combustion region. Complete combustion in these systems typically requires a combustion region length of at least fifty inches.
In order to reduce the length of the combustion region and provide a physically compact gas-fired burner and combustion region, one of the features needed is premixing of the air and fuel gas before entering the burner, or within the burner itself. For example, U.S. Pat. No. 5,975,887 describes a compact gas-fired burner having a premixing tube disposed within the burner and a fuel gas tube extending into the premixing tube.
Accordingly, there is a need to provide a power-driven gas-fired burner that provides a stable flame and complete combustion in a reduced length combustion region to reduce the overall size requirement of a water heating system in which it is employed. Such a burner should operate efficiently and reduce radiant transfer of energy from the flame directly to the walls.
This invention solves the deficiencies described in the previous section and provides a fuel gas burner having a short, stable flame to be used with a reduced length combustion region and chamber. The fuel gas burner of this invention can be used, for example, in a condensing, fully modulating, forced draft, vertical single-pass, fire-tube water heating system that operates over a broad modulation range with excellent stability, reliability and cost-efficiency.
These objectives and characteristics are achieved, in accordance with the present invention, by providing a novel combination of several components in one embodiment to form a compact fuel gas burner that produces a short, attached flame in an adjacent combustion region, the fuel gas burner having a recessed head, fuel gas channel design, multiple pathways for air and fuel gas to provide mixing, rotation of the combusting air-fuel gas mixture, and partial premixing of air and fuel in the burner head.
The fuel gas burner of this invention has a fuel gas pipe attached to a burner head. The burner head comprises a bottom plate concentrically attached to a middle plate, in which the bottom plate and middle plates define an opening at the center to receive fuel gas from the fuel gas pipe. The bottom plate and adjacent middle plate together define an annular cavity that is open at the center for receiving fuel gas from the fuel gas pipe and is closed at the outer edge of the middle plate. The outer edge of the middle plate and the adjacent portion of the bottom plate define a plurality of fuel gas channels extending radially outward. The bottom and middle plates further define at least two annularly spaced-apart holes for passage of air directly through the bottom and middle plates into the combustion region. The bottom plate also defines at least two annularly spaced-apart holes for passage of air into the annular cavity to mix with fuel gas in the cavity. A corresponding set of matching holes is provided in the middle plate to allow the partially premixed air and fuel gas to flow out to the burner head surface. The bottom plate and middle plate also have openings for inserting a flame detector and an igniter into the combustion region. The burner head further comprises a gas cap concentrically attached to the middle plate for closing the second end of the fuel gas pipe. The gas cap has at least two annularly spaced-apart holes for passage of fuel gas directly to the combustion region. The burner head also comprises annularly spaced-apart spinner vanes attached to the bottom plate extending away from the bottom plate toward the combustion region in asymmetric relation to the fuel gas channels.
The burner head resides in a shell comprising an annular baffle concentrically attached to a cylindrical side wall, the shell defining a shell chamber within which the burner head resides. The annular baffle has an opening through which the fuel gas pipe passes and an opening for entry of air into the shell chamber. The burner head and the cylindrical side wall define an annular opening for passage of air from the shell chamber around the burner head to the combustion region.
The shell and annular baffle are covered by a housing which attaches to and supports the shell. The housing defines an opening through which the fuel gas pipe enters the shell chamber and a spiraling air duct through which air enters the shell chamber. Openings are provided in the housing and annular baffle for the igniter and the flame detector.
The fuel gas burner of this invention provides stable and complete combustion of fuel gas within a distance of less than about twenty-four inches from the fuel gas burner head at energy release rates of up to and exceeding two million BTU per hour.
In another embodiment, this invention is an improved water heating system comprising a fuel gas burner as described, a combustion chamber for receiving the heated gas and combustion products of the fuel gas burner, a heat exchanger adjacent to the combustion chamber for providing heat transfer between the heated gas and combustion products of the fuel gas burner and a second fluid, and a temperature controller using at least one temperature sensor and at least one air/fuel valve.
In another embodiment, this invention is a method of heating water by providing a fuel gas burner according to this invention, installing the fuel gas burner in a water heating system, and operating the system to transfer heat to the water.
For a better understanding of the present invention and its objects, reference is made to the following drawings and description to be considered in light of the complete application, and the scope of this invention as pointed out in the appended claims.